[0001] This invention relates to an oil-in-water emulsion stabilized only by smectite clay
mineral which has excellent emulsion stability in a broad range of pH and salt concentration.
[0002] Emulsions are colloidal systems where droplets of one liquid are dispersed in a second
non-miscible liquid. Though they are thermodynamically unstable and tend to separate,
they can be stabilized by surfactants, amphiphilic molecules which adsorb at the liquid
interface and reduce the interfacial tension between the immiscible liquids. Alternatively,
solid particles can also be used to stabilize emulsions, provided that they fulfill
the partial wetting conditions, that means, the particles are wetted by both liquids
and therefore adsorb at the interface, leading to a reduction of the liquid interfacial
area of the system. Specific benefit derived from the stabilization of emulsions by
particles is the high resistance against coalescence due to the practically irreversible
attachment of the particles at the interface. This feature becomes of special interest
in application fields like enhanced oil recovery or agrochemical formulations, where
high temperature and/or salinity dominates and surfactants usually show a lack of
performance, as described in
WO-2014/114538 and
WO-2015/170099. Moreover, the formulation of "surfactant-free" emulsions allows to overcome adverse
effects related to the use of surfactants in certain applications, like skin irritancy
in topical formulations, as described in
International Journal of Pharmaceutics 428 (2012) 1 - 7.
[0003] Clay minerals have a sheet-like structure and are composed of tetrahedral layers
of silicate linked to octahedral layers of metal oxides. Depending on the number of
layers linked together building a structural motif, clays are classified in 1:1 phyllosilicates,
which consist of one tetrahedral layer of silica bond through oxygen atoms to one
octahedral layer of metal (aluminium or magnesium) oxide, 2:1 phyllosilicates, which
consist of octahedral metal (aluminium or magnesium) oxide layer sandwiched between
2 tetrahedral layers of silica, and 2:1 inverted ribbons, which present continuous
fibrous-like tetrahedral sheets together with discontinuous octahedral sheets.
[0004] Smectites are a group of 2:1 phyllosilicates characterized by the partial substitution
of cations in the layers by other cations of lower charge, thus originating an excess
of negative charge in the layers which is compensated by cations intercalated between
the sheets. Typically the degree of substitution and therefore of charge is low for
the members of the smectite family. A further differentiation of smectites is done
regarding either the oxygen atoms in the octahedral oxide layer are coordinated bridging
two metal centers (dioctahedral smectites, the metal is typically Al
3+) or each oxygen atom is shared by three metal centers (trioctahedral smectites, the
metal is typically Mg
2+).
[0005] Montmorillonite (Z
X[Al
23+]
oct[Si
4]t
etrO
10(OH)
2) is the main representative class of the dioctahedral smectites. It consists of an
octahedral layer of aluminum oxide sandwiched between two silica tetrahedral lays
platelet-like monocrystals with a constant thickness of 1.1 nm and a diameter between
50 - 1000 nm. The platelets are typically stacked in large piles. The substitution
of aluminium atoms by magnesium atoms in the octahedral layer or silicon atoms by
aluminum atoms in the tetrahedral layer creates a net negative charge in the crystals,
which is balanced by the presence of counter ions (typically sodium or calcium) in
the interlayer gallery. (Z
X[Mg
32+]
oct[Si
4]t
etrO
10(OH)
2) is the basic structure of trioctahedral smectites. When the magnesium ions are partially
substituted by lithium ion, the clay is called laponite (or hectorite), while when
the tetrahedral silicon are partially replaced by aluminium, the clay is called saponite.
[0006] The cation exchange capacity of a clay (CEC) is a well-known parameter used to define
the concentration of negatively charged sites on the clay sheets that can adsorb exchangeable
cations. The higher the CEC value, the higher the anionic charge on the clay crystals.
Typically, the CEC can be determined using the ammonium chloride method, as described
for example in
Clay Minerals, 44 (2009), 525 - 537 and is indicated in terms of miliequivalents per 100 g of dry clay.
[0007] FR-2976503 discloses oil-in-water emulsions stabilized by clay minerals previously treated with
organic compounds in order to render the particles more hydrophobic and thus increase
their affinity towards the oil phase. The pre-treatment of the clays with the organic
compounds is an additional processing step. For environmental and economic reasons
it would be of interest to find a clay which can stabilize oil-in-water emulsions
without need of a hydrophobizing pre-treatment with organic compounds.
[0008] The stabilization of mixtures of monolinolein and different kind of oils (glycerol
trioleate, R-(+)-limonene and styrene) by two unmodified smectites (purified Na
+-montmorillonite and synthetic laponite) is described in
Journal of Colloid and Interface Sciences, 333 (2009) 563-569. The clays are claimed to act as cosurfactants in the sense that they support monolinolein,
which is an amphiphilic molecule and act as emulsifier, to stabilize the oil-in-water
emulsions. Either the emulsions can be stabilized in the absence of surfactant solely
by the clays or the stability of the described emulsions is perturbed by the addition
of electrolyte to the aqueous phase is not disclosed.
[0009] Stable emulsions of paraffin oil in water stabilized by combinations of smectites
and non-ionic surfactants are disclosed in
Applied Clay Science, 14 (1999) 83 - 103. Especially Na
+ montmorillonite with a cation exchange capacity of 103 meq per 100 g dry clay is
mentioned to form stable emulsions. The addition of sodium chloride or calcium chloride
to the aqueous phase is reported to stabilize the emulsions against coalescence. No
information about the ability of the clays to stabilize the emulsions in absence of
surfactants is disclosed.
[0010] Nevertheless there is an on-going need to provide solid materials able to stabilize
oil-in-water emulsions in the absence of amphiphilic molecules. Such materials have
to be economically competitive and environmentally sustainable, that means they must
be easy to be produced and do not be classified as hazardous chemicals.
[0011] Unexpectedly it was found that natural clays which belong to the class of saponites
or are natural blends of saponite and kerolite and have a cation exchange capacity
equal or lower than 40 meq per 100 g of dry clay are able to stabilize oil-in-water
emulsions without addition of any surfactant.
[0012] The clays disclosed in this invention are natural raw materials which due to their
unexpected unique surface properties can efficiently adsorb at the oil-water interface
and prevent oil droplets from coalescence without the need of a hydrophobizing pre-treatment
with organophilic molecules.
[0013] Surprisingly, the clays being object of this invention stabilize oil-in-water emulsions
without additional stabilizers such as surfactants being needed. Oil-droplet stabilization
in the aqueous phase and prevention of coalescence and phase separation is fulfilled
by the clays disclosed in this invention.
[0014] Furthermore it was also unexpectedly found that the clays disclosed in this invention
are able to stabilize oil-in-water emulsions against coalescence at diverse pH values
and salt concentrations.
[0015] The oil-in-water emulsion is considered to be stable if coalescence and phase separation
does not occur for at least three months after preparation during storage at temperatures
around room temperature (23 °C). Coalescence can be tracked by changes in the droplet
size during time and phase separation can be visually evaluated.
[0016] The present invention is related to oil-in-water emulsions stabilized solely by natural
clays which belong to the class of saponites or are natural blends of saponite and
kerolite and have a cation exchange capability equal or lower than 40 meq per 100
g of dry clay.
[0017] The instant invention therefore relates to a composition in form of a stable oil-in-water
emulsion comprising
- a) at least one natural clay which belongs to the class of saponites clay minerals
and have a cation exchange capability equal or lower than 40 meq per 100 g of dry
clay,
- b) a dispersed oil component, and
- c) an aqueous continuous phase
[0018] Natural clays are extracted from geological deposites. Natural clays occur typically
as a mixture of diverse components, such as clay minerals, metal oxides and other
weathered minerals. Preferred natural clays according to the present invention belong
to the class of the saponites. Most preferred clays according to the present invention
occur as a natural blend of saponite and kerolite.
[0019] Preferred clays (component a) suitable to formulate compositions in accordance with
the present invention have a cation exchange capacity from 1 to 35 meq per 100 g of
dry clay. Particularly preferred clays have a cation exchange capacity from 15 to
35 meq per 100 g of dry clay.
[0020] The clays have preferably a BET-surface area from 90 to 300 m
2/g. Particularly preferred clays have a BET-surface area from 105 to 275 m
2/g. Most preferred clays have a BET-surface area from 120 to 250 m
2/g.
[0021] Preferred clays have a surface free energy from 10 to 50 mN/m. Particularly preferred
clays have a surface free energy from 20 to 45 mN/m.
[0022] The concentration of clay in the composition in accordance with the present invention
is preferably from 0.01 % to 15 % by weight, more preferably from 0.1 % to 10 % by
weight and most preferably from 0.3 % to 5 % by weight.
[0023] The composition of the present invention contain at least one oil component (component
b), which is any fatty substance which is liquid at room temperature (25 °C). Among
the oils that can be used in the emulsion of the invention, mention may be made, for
example, of plant oils such as sunflower, olive, soybean, rapeseed, canola, jojoba,
palm, peanut, avocado, soft-almond, apricot and corn oils and the liquid fraction
of karite butter; chemically modified plant oils such as methylated or ethylated seed
oils and medium-chain triglycerides; hydrocarbon-based oils chosen from linear or
branched hydrocarbons of mineral or synthetic origin, preferably from a volatile or
nonvolatile liquid paraffin oil, hydrogenated isoparaffin, naphthalene oil, a totally
or partially hydrogenated liquid polydecene, isoeicosane, a decene/butene copolymer,
or a polybutene/polyisobutene copolymer, and mixtures thereof. Synthetic oils such
as 2-ethylhexyl palmitate, isopropyl myristate, isononyl isononanoate and cetearyl
octanoate; volatile or non-volatile silicone oils and fluorinated oils.
[0024] The concentration of oil component in the composition in accordance with the present
invention is preferably from 0.5 % to 60 % by weight, more preferably from 1 % to
45 % by weight and most preferably from 5 % to 30 % by weight.
[0025] For the skilled person in the art results obvious that liphophilic active ingredients
can be dissolved in the oil phase and be thus formulated in form of a stable oil-in-water
emulsion in accordance with the presence invention. Among the active ingredients that
can be formulated, mention may be made, for example, of cosmetic ingredients such
as essential oils, UV-filters and anti-aging agents, and agrochemical agents such
as pesticides.
[0026] In the aqueous phase (component c) of the emulsion different additives can be present,
such as rheology modifiers, pH control agents, chelating agents, electrolytes, biocides
and humectants.
[0027] The oil-in-water emulsions solely stabilized by clays according to this invention
are stable in a broad range of pH and electrolyte concentration. Stable means that
there are almost no changes in the droplet size of the dispersed oil phase when pH
and/or salt concentration in the continuous aqueous phase of the emulsion is varied,
and no phase separation is observed.
[0028] In the compositions of the present invention, typically there is no need for the
presence of conventional emulsifiers in the form of amphiphilic molecules in order
to achieve stabilization of the dispersed oil phase. If used, the amphiphilic molecules
are present in an amount of at most 0.5 % by weight and most preferably at most at
the critical micellar concentration, and they are incorporated in the formulation
to optimize the wettability of the emulsion by lowering of the dynamic surface tension
of the system.
[0029] Another embodiment of the present invention is also related to a process for the
production of an oil-in-water emulsion with a composition according to the described
above. The emulsion can be prepared by using any of the emulsification techniques
described elsewhere, such as for example, high pressure homogenization, rotor-stator
mixing, cavitation, membrane emulsification and microfluidic techniques. In a preferred
manufacturing process, the clay (component a) is dispersed in water under high mechanical
shear generated by a rotor-stator mixer. The oil phase is added to the aqueous clay
dispersion and the composition is homogenized with a rotor-stator mixer in order to
obtain a stable oil-in-water emulsion.
[0030] The preferred droplet size of the emulsions can range from 1 µm to 200 µm. Specially
preferred droplet size of the emulsion is in the range from 10 µm to 150 µm. Most
preferred droplet size of the emulsion is in the range from 15 to 100 µm. This droplet
size refers to the discontinuous phase of the emulsion and is determined by analysis
of microscopic image of the emulsion.
[0031] Other embodiment of the present invention is related to the use of a composition
according to the described above in personal care and cosmetics formulations, such
as for example, hand creams, lotions, hair shampoos and conditioners and sunscreens;
in agrochemical formulations, in emulsion paints, in cleaning formulations and in
the oil & mining industry.
Examples
Characterization of the clays
[0032] The cationic exchange capacity (CEC) was determined following the NH
4Cl method. The clay material was dried at 150 °C for 2 h and the resulting material
allowed to react under reflux with a large excess of aqueous NH
4Cl solution for 1 h. After standing at room temperature for 16 h, the material was
filtered. The filter cake was washed, dried and ground, and the NH
4 content in the clay material was determined through measurement of the N content
by elemental analysis using a CHN-Analyser Vario EL III by Elementar, Hanau (D).
[0033] The specific surface area was determined by the BET method (single-point method using
nitrogen, according to DIN 66131) with an automatic nitrogen porosimeter by Micrometrics,
type ASAP 2010.
[0034] The surface free energy was calculated applying the Owens-Wendt-Rabel-Kaelbe model
using contact angles for water and diiodomethane obtained from powder contact angle
measurements on the clays with a Krüss Force Tensiometer K100 following the Washburn
method. For clays swelling rapidly in contact with water, ethanol was used instead
of water for the contact angle measurements.
Clay |
CEC (meq in 100 g) |
BET-surface area (m2/g) |
Surface free energy (mN/m) |
Mineral class |
A |
25 |
125 |
24 |
natural saponite |
B |
20 |
224 |
44 |
natural blend of saponite with kerolite |
C (comparison) |
52 |
239 |
55 |
natural blend of montmorillonite with amorphous silica |
D (comparison) |
94 |
83 |
48 |
natural montmorillonite |
E (comparison) |
49 |
67 |
|
natural montmorillonite |
General procedure for preparation and characterization of emulsions
[0035] Clay particles are firstly dispersed in buffer solution by stirring at 800 rpm for
5 min with high shear mixer (Ultra Turrax T25-IKA
® equipped with dispersing tool consisting of S25N shaft and 25G generator) and then
the oil phase is added to the dispersion and homogenized at 6500 rpm for 5 min. The
total mass of the emulsion is typically 100 g.
[0036] The emulsion type was determined by the drop test. A drop of the formed emulsion
was added to water. Immediate dispersion of the droplets confirmed the presence of
oil-in-water emulsion type.
[0037] The stability of the emulsion was evaluated from the visual inspection and assessed
according to the volume of emulsion formed and the speed of phase separation after
preparation and after 90 days at room temperature. According to the volume of each
phase (especially the oil separation), the stability of the emulsion was assessed.
The more oil release pointed to the instability of the emulsion.
[0038] The morphology and size of the emulsion droplets was observed by optical microscope
(Olympus DP26, Japan) and the droplet size determined by analysis of microscopic image
of the emulsion.
[0039] For comparison of the emulsification efficiency, clays A, B, C, D and E were used
at a concentration of 1 wt.-% in citrate buffer pH 5.5 to prepare emulsions of Myritol
318 (medium chain triglyceride, BASF) at a volume fraction of 20 %. The volumen of
the existing phases (separated oil phase, (creamed) emulsion, aqueous phase and solid
sediment) was recorded after preparation (D1) and after 90 days storage at room temperature
(D90)
|
A |
B |
C |
D |
E |
|
D1 |
D90 |
D1 |
D90 |
D1 |
D90 |
D1 |
D90 |
D1 |
D90 |
oil phase (vol.-%) |
0 |
0 |
0 |
0 |
7 |
13 |
48 |
48 |
2 |
4 |
emulsion (Vol.-%) |
81 |
56 |
49 |
40 |
34 |
31 |
0 |
0 |
37 |
33 |
aqueous phase (vol.-%) |
18 |
44 |
40 |
51 |
52 |
49 |
52 |
12 |
61 |
48 |
sediment (vol.-%) |
1 |
1 |
11 |
9 |
7 |
7 |
0 |
40 |
0 |
15 |
[0040] Clays A and B form very stable emulsions and no separation of oil was observed after
90 days at room temperature. Compression of the emulsion volumen due to creaming occured
over the storage time which was due to the density differences between the bulk and
the dispersed phases, but the oil droplets remained dispersed indicating that the
emulsion was stable.
[0041] For comparison, clay C and E showed oil separation after preparation, indicating
that these clays were not as efficient for emulsion stabilization as A and B. Moreover,
the volume of separated oil phase increased during storage. Clay D was not able at
all to stabilize oil droplets and phase separation occurred after preparation.
[0042] To check the influence of salt on the stability of the emulsions, samples were prepared
using the same conditions as mentioned before but adding increasing concentrations
of sodium chloride. The droplet size of the resulting emulsions was compared.
NaCl (wt.-%) |
Average droplet size (µm) |
A-1753 |
B-1694 |
E-0075 |
0 |
25 |
45 |
107 |
0.5 |
25 |
46 |
170 |
1.0 |
24 |
47 |
342 |
[0043] The droplet size of emulsions prepared with clays A and B remained almost invariable
independently of the salt concentration in the aqueous phase, whereas the droplet
size increased notably with the salt concentration for clay E (see figure 1).
[0044] The same kind of experiment was repeated but changing the pH value, which was adjusted
to 4 and 9 using respective citrate buffers. Again clays A and B were able to stabilize
the emulsions at both acidic and alkaline pH, whereas clay E was not able to form
any emulsion at pH 9 and phase separation was immediately found after preparation.
[0045] In order to show the ability of the clays according to this invention to stabilize
emulsions of diverse types of oils, quick emulsification tests were done by dispersing
clay A in citric buffer pH 5.5 by vortex mixing (IKA 4 Digit) at 3000 rpm for 20 s,
adding the oil and further mixing for 40 s at the same speed. Clay concentration was
1 wt.-% and the oil volumen fraction 20 %. Oils used were Solvesso 200ND (mixture
of aromatic hydrocarbons, Exxonmobil), Canola based methylated seed oil (Oleon), tetradecane
(Alfa Aesar) and sunflower oil. Stable emulsions were obtained for all type of oils
(see figure 2).
[0046] Furthermore, in order to show that the ability of stabilizing oil-in-water emulsions
by the clays according to this invention is not affected by other typical components
of a formulation, such as for example rheology modifiers, biocides and humectants,
an oil-in-water emulsion containing 1 wt.-% of clay A, 20 wt.-% of medium chain triglyceride
(Myritol 318, BASF), 0.5 wt.-% of a copolymer of vinylpyrrolidone and 2-acrylamido-2-methylpropane
sulfonic acid (Aristoflex AVC, Clariant), 1 wt.-% of phenoxyethanol (Phenonip ME,
Clariant) and 2.5 wt.-% glycol (Merck) was prepared as follows:
Glycerol was dissolved in desionized water and clay A was added and stirred at 800
rpm for 5 min with high shear mixer (Ultra Turrax T25-IKA® equipped with dispersing tool consisting of S25N shaft and 25G generator). Then Myritol
318 was added to the dispersion and homogenized at 6500 rpm for 5 min. Aristoflex
AVC was then added and the mixture was homogenized for another 2 min at 9500 rpm.
Finally, Phenonip ME was added to the emulsion and gently stirred for 1 min.
[0047] The obtained emulsion had the texture of typical cosmetic hand cream. The stability
of the emulsion was investigated by comparison of the rheological properties of the
sample after preparation and after 90 days storage at 50 °C.
[0048] All the measurements were done with a rheometer MARS III (Thermofisher Scientific)
with a cone (2°, 35 mm) / plate geometry.
[0049] The flow of the emulsion was evaluated by a shear controlled sweep between 0 and
1000 1/s at 23 °C. Frequency sweeps were carried out at 23 °C to characterize the
viscoelastic properties of the emulsion. After pre-shearing steps at constant shear
rate of 100 1/s for 2 min followed by 0.001 1/s for 30 min, a 0.01592 - 15.92 Hz Hz
ramp at constant strain of 1 % (which was preciously determined to be in the linear
viscoelastic range via strain controlled amplitude sweep) was measured.
[0050] The measurements showed that the rheological properties of the emulsion remained
almost constant during the storage, indicating that the system was highly stable and
no changes occurred in the colloidal structure of the emulsion (see figure 3).
1. Composition in form of a stable oil-in-water emulsion comprising
a) at least one natural clay which belongs to the class of saponites clay minerals
and has a cation exchange capability equal or lower than 40 meq per 100 g of dry clay,
b) a dispersed oil component, and
c) an aqueous continuous phase.
2. Composition according to claim 1, wherein the natural clay is a natural blend of saponite
and kerolite.
3. Composition according to claim 1 and/or 2, wherein the cation exchange capacity is
from 1 to 35 meq per 100 g of dry clay.
4. Composition according to one or more of the preceding claims, wherein the clay has
a BET-surface area from 90 to 300 m2/g.
5. Composition according to one or more of the preceding claims, wherein the clay has
a surface free energy from 10 to 50 mN/m.
6. Composition according to one or more of the preceding claims, wherein the concentration
of clay in the composition is in the range from 0.01 % to 15 % by weight.
7. Composition according to one or more of the preceding claims, wherein the oil component
(b) is a fatty substance which is liquid at room temperature (25 °C).
8. Composition according to claim 7, wherein the oil component is made of plant oils,
in particular sunflower, olive, soybean, rapeseed, canola, jojoba, palm, peanut, avocado,
soft-almond, apricot and corn oils and the liquid fraction of karite butter or chemically
modified plant oils, hydrocarbon-based oils or synthetic oils or volatile or non-volatile
silicone oils and fluorinated oils.
9. Composition according to one or more of the preceding claims, wherein the concentration
of oil component in the composition is in the range from 0.5 % to 60 % by weight.
10. Composition according to one or more of the preceding claims, wherein liphophilic
active ingredients are dissolved in the oil phase.
11. Composition according to one or more of the preceding claims, wherein the aqueous
phase (c) comprises different additives, in particular rheology modifiers, pH control
agents, chelating agents, electrolytes, biocides and/or humectants.
12. Composition according to one or more of the preceding claims, wherein the droplet
size of the emulsions is in the range from 1 µm to 200 µm.
13. Use of a composition according to one or more claims 1 to 12 for personal care and
cosmetics formulations, in agrochemical formulations, in emulsion paints, in cleaning
formulations and in the oil & mining industry.